1. Field of the Invention
The present invention relates to phase locked loops, more particularly to acquisition aids in phase locked loops.
2. State of the Art
A phase locked loop (PLL) may be inadvertently forced to function outside its operating range because of a variety of factors. In some cases, once functioning outside of its operating range, the PLL may not be able to be driven back into the proper range even when system inputs suggest that it should. For example, within a PLL, a frequency pre-divider may be unable to function when its input, the voltage controlled oscillator (VCO) output of the PLL, reaches a very high frequency. The VCO's output may be operating at a very high frequency because the PLL has been incorrectly programmed. Nevertheless, when the PLL is subsequently reprogrammed, it is unable to recover and operate normally.
There are other cases in which the PLL can be inadvertently forced to function outside of its operating range. In the case described above, the errant behavior of the PLL caused the VCO to generate the highest frequency it could provide. In a different PLL architecture, the behavior of the circuit may be to produce the lowest frequency the circuit can provide.
What is needed, then, is a way to ensure that if a PLL does exceed its operating range, it is able to recover.